1. Field of the Invention
The present invention relates to photographic printers. In particular, the present invention is related to apparatus for controlling exposure times in a subtractive type photographic printer.
2. Description of the Prior Art
Photographic printers produce color or black-and-white prints or transparencies from photographic film originals (generally negatives). High intensity light is passed through the film and imaged on the photosensitive print medium. The photographic emulsion layers on the print medium are exposed and subsequently processed to produce a print of the image contained in the original.
Nearly all photographic printers used commercially today are "subtractive" type printers. A subtractive printer typically includes a source of white light, a shutter, and cyan, magenta and yellow color filters to terminate exposure in the red, green and blue color channels, respectively. When the shutter is opened, white light is permitted to pass through the negatives and is imaged on the photosensitive medium. When the exposure in the red color channel is completed, the cyan filter is driven into the light path, so as to subtract the red component of the light. Similarly, the magenta filter is driven into the light path to terminate a green exposure, and the yellow filter is driven into the light path to terminate the blue exposure.
In order to increase efficiency and minimize the time required to fill customer orders, there has been continuing efforts to increase the speed of photographic printers. One technique which has been used to increase the speed of photographic printers has been to increase the intensity of the light used in printing (and thus decrease exposure times). Since the exposure of the print medium is a function of light intensity and exposure time, in general the exposure time can be reduced by increasing intensity, while still yielding the same exposure.
While the increase in intensity of the light used for printing has in general resulted in higher speed operation of photographic printers, there are problems which have been encountered. In particular, highly underexposed negatives have a density which is very low, and the exposure time required to produce the desired exposure is very short. As the light intensity is increased, the exposure time required for proper printing becomes extremely short. Since solenoids are typically used to drive the shutter out of the light path and the color filters into the light path, the minimum exposure time in prior art photographic printers has been limited by the response time of the shutter and filter drive solenoids. The increase in print light intensity has been limited, therefore, by this minimum exposure time.
In the past, attempts have been made to overcome the problem of underexposed negatives having exposure times less than the minimum exposure time. One technique which has been used is known as "dual intensity printing". In this technique, most negatives use the normal high intensity light. For underexposed negatives which would have an exposure time falling below the minimum exposure time if the normal high intensity light is used, the light intensity is reduced to a lower intensity by introducing a neutral density paddle in the light path. This increases the exposure time to above the minimum exposure time.
The dual intensity printing technique, however, adds complexity to the printer. It requires an additional neutral density paddle having a design which will not affect the intensity distribution of the light and yet will decrease the intensity to a desired level. In addition, a solenoid or other drive apparatus must be provided to control the position of the neutral density paddle. The exposure control of the printer must also be modified to identify those negatives having an exposure time less than the minimum exposure time and to actuate the neutral density paddle solenoid when printing those identified negatives.